Medicines, synthesis reagents, food additives and the like are synthesized through several reaction processes. During the reaction processes, by-products and decomposed products are formed. In order to remove those, crystallization is carried out in the final process. However, when the content of the impurities is large, or the properties of the impurities are similar to those of a desired chemical compound, sometimes it is impossible to remove the impurities by one crystallization process. Accordingly, methods have been known wherein prior to the crystallization, the chemical compound solution undergoes activated carbon treatment or activated clay treatment. However, depending on the kind of the chemical compound and the amount of the impurities, the impurities have not been sufficiently removed and loss has been caused because the desired chemical compound is adsorbed.
Furthermore, in the crystallization, the filterability of slurry as well as the purification efficiency is important in terms of the ease of process operation. As crystallization methods which aim at the improvement in the filterability, have been known one method wherein seed crystals are added and another wherein at the initial stage of crystallization, a small amount of crystals are produced and are employed as crystal seeds. However, depending on chemical compounds, no improvement in the filterability has been obtained and sometimes the quality has been degraded. Furthermore, there have been methods wherein by the addition of a poor solvent or cooling operation, crystal size and shape are controlled. According to this method, depending on chemical compounds, no effect has been obtained and sometimes, the quality has been degraded.
As mentioned above, various crystallization methods have been used in regard to the improvement in quality, filterability and yield. However, it has been difficult to meet all the requirements for quality, yield and filterability at the same time.